remove persist.sys.usb.config override

The variable persist.sys.usb.config is no longer honored by the framework,
and it's presence causes unnecessary toggling of the USB driver, which
disonnects ADB and makes the device's connection unstable.

Delete it.

Bug: 21404762
Bug: 18905620
Change-Id: Ic8d7b003df07190d2f1f93a332154f4d649af2e6
diff --git a/device.mk b/device.mk
index ef1ccbd..4db414f 100644
--- a/device.mk
+++ b/device.mk
@@ -216,10 +216,6 @@
 # setup dalvik vm configs.
 $(call inherit-product, frameworks/native/build/tablet-10in-xhdpi-2048-dalvik-heap.mk)
 
-# set default USB configuration
-PRODUCT_DEFAULT_PROPERTY_OVERRIDES += \
-    persist.sys.usb.config=mtp
-
 # for off charging mode
 PRODUCT_PACKAGES += \
     charger \